Use of 2D NMR Methods in Imaging

We saw in Section 3.7 that in ordinary NMR spectroscopy the rate of data acquisition determines the maximum spectral range. For imaging it is clear from Eq. 14.2 that for a given value of Gx the spectral width determines the maximum range, Fx, that can be observed in the x direction. Fx is called the field of view (usually abbreviated as FOV) in the x direction. 

Equation 14.2 applies to a sample that contains only a single spectral line. When chemical shifts are taken into account, it becomes 

FIGURE 14.2 Pulse sequence for a two-dimensional NMR image in the xy plane, with slice selection along z. The value of the phase-encode gradient Gx is altered in successive repetitions, as indicated. The rf pulses are frequency selective and define the slice while gradient Gz is applied. Within one value of Gx there may be several repetitions with different values of frequency to obtain data from multiple slices. Data are acquired from the spin echo while the readout gradient Gy is applied. 

The rf portion of Fig. 14.2 shows a 90°, r, 180° spin echo pulse sequence, rather than a simple 90° pulse. All imaging studies employ either a spin echo sequence or a gradient echo to avoid acquisition of data during the FID, which decays rapidly in the presence of a magnetic field gradient. Instead, data acquisition occurs during the echo, when the rf circuitry is not subject to aberrations 

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